The present invention relates to the utilization of solenoid operated valves for pressure control in hydraulically operated systems where the solenoid is commanded by an electronic controller responsive to system operating parameter inputs. An example of such use of a solenoid operated pressure control valve is that employed for controlling the shifting or gear set changes in a motor vehicle automatic speed change transmission.
In such vehicle automatic speed change transmission applications, the solenoid operated pressure control valves are typically used to control the hydraulic fluid pressure supplied from a pump in the transmission to a hydraulic circuit connected to an actuator usually of the piston operated type for actuating a shifting clutch, typically a band clutch. Thus, a shifting pattern can be programmed into an electronic controller which is connected to energize a solenoid operated pressure control valve for actuating each of the shift band clutches in the transmission associated with the various gear sets. This arrangement has proven to be generally satisfactory; however, very precise control of the pressure to the clutch actuator is often required in order to effect smooth speed changes. This is particularly true where it is required to engage one band clutch for a non-driving gear set while the band clutch for a driving gear set is being disengaged. This requirement for precise pressure control to the band clutch actuators in an automatic transmission has further required precise control of the moving valve element in the solenoid operated valve in response to the energization current from the controller to the solenoid coil. Inasmuch as the shift control in the transmission is of the open loop type, the solenoid operated valve must be calibrated accurately and once calibrated is required to maintain the calibration throughout its service life. Furthermore, the effects of hysteresis in the solenoid valve occurring between increasing and decreasing current has proven to be troublesome and has resulted in prohibitive variation in the pressure output of the valve for the particular command signal or energization current.
Accordingly, it has been desired to find a way or means of improving the pressure output accuracy of a solenoid operated pressure control valve and to find a way of calibrating the valve in a manner which will assure accurate valve output in response to a command pressure energization current. It has further been desired to provide a way to improve the calibration and output of a solenoid operated pressure control valve in a manner which is easy to perform during valve manufacture and cost effective in high volume production such as for motor vehicle transmission applications.